Several prior art techniques have been proposed in the art for the high speed reproducible marking of thermoplastic articles using laser irradiation. In general, the prior art proposals include incorporating a material which is capable of selectively absorbing laser irradiation so as to locally heat the thermoplastic and thereby induce localized thermal change, for example, through melting, evaporation or carbonization, in the article's surface.
For example, Japanese Patent Publication No. 1495/1981 discloses a laser-marking technique whereby a dye and a silicon-containing inorganic compound or a silicon-containing dye are incorporated into the material to be molded; Japanese patent Publication No. 118926/1984 discloses a method whereby a radiation-absorbing substance, such as a metal silicate is added to the molding material; Japanese Patent Publication No. 187050/1984 discloses a technique whereby at least 20% of hydrated alumina is added as an organic filler; Japanese Patent Publication No. 204888/1990 discloses a technique whereby a pigment containing a phosphate is added to the molding material; Japanese Patent Publication No. 48984/1990 discloses a technique whereby a non-black inorganic lead compound is incorporated into the molding material; Japanese Publication No. 10884/1991 discloses incorporating a non-white metal titanate into the molding material; Japanese Publication No. 155493 discloses incorporating a black organic dye into the molding material; and Japanese Patent Publication No. 166488/1985 discloses incorporating a metal hydroxide and/or a water-containing metal compound and a colorant.
As can be appreciated, while incorporating a variety of materials into the thermoplastic to be laser-marked may result in adequate marking characteristics, there is a risk that the additive which is incorporated into the thermoplastic so as to enhance its laser-marking properties could deleteriously affect the thermoplastic's inherent and advantageous physical properties. Furthermore, the incorporation of such additives into a thermoplastic resin typically results in a more complex (and costly) compounding procedure.
What has been needed in the art, therefore is a laser-marking process for thermoplastic resins whereby the addition of special additives for laser marking may be obviated. It is towards fulfulling such a need that the present invention is directed.
In this connection, the present invention relates to processes for laser-marking surfaces of thermoplastic articles whereby at least the surface region of the thermoplastic article has a critical oxygen index of 22% or above as determined by ASTM D2863. More specifically, it has been found that very sharp, unambiguous black markings (such as indicia, characters, symbols, patterns and the like) may be made with high speed and with reproducible reliability by means of laser irradiation if at least the surface of the thermoplastic article to be marked has a critical oxygen index according to ASTM D2863 of 22% or above.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follows.